Flame spraying involves the heat softening of a heat fusible material, such as a metal or ceramic, and propelling the softened material in particulate form against a surface which is to be coated. The heated particles strike the surface and bond thereto. A conventional flame spray gun is used for the purpose of both heating and propelling the particles. In one type of flame spray gun, the heat fusible material is supplied to the gun in powder form. Such powders are typically comprised of small particles, e.g., below 100 mesh U.S. standard Screen size to about 5 microns.
A flame spray gun normally utilizes a combustion or plasma flame to produce the heat for melting of the powder particles. It is recognized by those of skill in the art, however, that other heating means may be used as well, such as electric arcs, resistance heaters or induction heaters, and these may be used alone or in combination with other forms of heaters. In a powder-type combustion flame spray gun, the carrier gas for the powder can be one of the combustion gases or an inert gas such as nitrogen, or it can be simply compressed air. In a plasma spray gun, the primary plasma gas is generally nitrogen or argon. Hydrogen or helium is usually added to the primary gas. The carrier gas is generally the same as the primary plasma gas, although other gases, such as hydrocarbons, may be used in certain situations.
The material alternatively may be fed into a heating zone in the form of a rod or wire. In the wire type flame spray gun, the rod or wire of the material to be sprayed is fed into the heating zone formed by a flame of some type, where it is melted or at least heat-softened and atomized, usually by blast gas, and thence propelled in finely divided form onto the surface to be coated. The rod or wire may be conventionally formed as by drawing, or may be formed by sintering together finely divided material, or by bonding together finely divided material by means of an organic binder or other suitable binder which disintegrates in the heat of the heating zone, thereby releasing the material to be sprayed in finely divided form.
Flame sprayed ceramic coatings containing refractories such as zirconium oxide are often used for thermal barrier protection of metal components, such as in gas turbine engines. The zirconium oxide may contain some hafnium oxide and incidental impurities. It typically is stabilized with calcium oxide or yttrium oxide or may be in the form of magnesium zirconate. Yttrium oxide is a preferable stabilizer because it renders long term stability at high temperatures. Such zirconium oxide coatings are generally used for thermal barrier purposes such as in gas turbine engines, requiring low thermal conductivity as well as resistance to thermal shock, hot corrosion and erosion.
Flame sprayed ceramic coatings usually are not fully dense, having some porosity typically up to about 20% depending on composition, powder size distribution, flame spray method and parameters. A higher porosity generally contributes to lower thermal conductivity and a higher degree of resistance to thermal stress than the denser coatings. However, a more porous coating will have lower resistance to corrosion and erosion and other wear conditions that exist in the environments where such coatings are used.
U.S. Pat. No. 4,328,285 describes plasma spraying spherical agglomerate particles formed by spray drying a two component powder of zironium oxide and at least 15% cerium oxide particles. An example teaches 26% cerium oxide. The patent is directed to improved resistance at elevated temperatures to vanadium impurities often present in turbine fuels. In this regard yttrium oxide is considered to be detrimental, and the patent explicitly excludes yttrium oxide as well as calcium oxide from the composition of the spray powder.
In view of the foregoing, it is a primary object of the present invention to provide a novel flame spray material for producing a ceramic coating characterized by low thermal conductivity.
It is another object of this invention to provide a novel flame spray material for producing a ceramic coating that has the combined properties of low thermal conductivity and high resistance to thermal shock, hot corrosion and erosion.
A further object of this invention is to provide an improved flame spray process for producing a ceramic coating characterized by low thermal conductivity.